We have expansive clays in our area, though not nearly as expansive as the soils found in the neighboring provinces of Saskatchewan and Manitoba where heaving of six or seven inches has been reported.
A cushion will not function adequately unless its thickness is at least twice the expected heave due to either frost or swelling soil or, alternatively, unless the grade beam is reinforced for the uplift pressure and is tied down at supports to resist the negative reactions.
I typically use 4" GeoSpan in the Edmonton area with no problems so far. For a project in Winnipeg, Manitoba, I used 6" GeoSpan and have heard of no problems to date. Grade beams were designed for a substantial uplift pressure in the event that the soil swelling exceeded the estimates. This could be considered double kill but it didn't really matter because the cost of the reinforcement was not significant.
In the 1950's, it was common to form grade beams with a 'V' shape on the bottom with the idea that the clay soil would cleave to each side when it heaved, hence prevent uplift. I never could understand that logic, but I cannot criticize it too much because some of those old buildings are still around today and are behaving reasonably well.